Rubys name can probably be traced from her skin color, which is pink (like Tria's and Sue's.) Also, Ruby is an easy to remember name that can be marketable, which is important for this character, as she is the only one in the Gang of Seven that has no previous appearance in the movies. I wonder how she and Chomper will be put into the 13th movie, assuming that they appear in it. So far, we have seen continuity between the 12th movie by the appearances of Tria and Tricia, so I suppose the order of events will be placed by the references to specific events in the past, as well as the release date in the real world. It's important for us to remember that when we see the 12th movie for the first time, those of us who have seen the tv series will actually be looking at past events (from our perspective.) Anyway, that's where Ruby's name comes from; the gemstone, or at least its color.
EDIT: The simplest way to set the internal chronology of the series in relation to any future movies and keep it all consistant would be to have a movie create a new set of conditions (like a new child, a departure, an arrival, or even a death), and to have those conditions appear in the series after a certain point. This would be facilitated best by having a movie appear in between seasons in the tv series. A possible example of this would be: Season 1 has no Ali in it, season 1 ends. Between seasons one and two a movie is released in which Ali could be made to be a permanent member of the valley's population. Season 2 begins, and Ali appears. We will know that everything that has happened in season 2 occured after the movie due to Ali's appearence. This has already happened once before; we know that the series takes place after movie twelve due to the appearance of Tricia. From this point of view, the tv series could be the best thing that ever happened to the movie series, as it encourages plot development and the appearance of new, permanent characters (Tria, Tricia, Ruby, Redclaw, Screech), and reappearances of old characters (Chomper, possibly others in the future.)